The present invention relates to an improved process for the complete removal of monomer residues present in acrylonitrile polymers manufactured according to processes wherein the polymers are obtained in the form of particles.
It is well known to polymerize and copolymerize acrylonitrile with the aid of a source of free radicals, the process being carried out in an aqueous medium. The polymer is insoluble in the reaction medium. As a rule, the polymerization is stopped well before the degree of conversion of the monomers reaches 100%. Accordingly, the polymer is obtained in the form of particles dispersed in an aqueous medium which still contains substantial amounts of acrylonitrile and, where relevant, comonomers, which have not reacted.
U.S. Pat. No. 3,553,248, issued to Wakita et al on Jan. 5, 1971 and assigned to Asahi Kasei Kogyo Kabushiki Kaisha, and U.S. Pat. Nos. 3,663,631 and 3,663,632, both issued to Takeya et al on May 16, 1972 and assigned to American Cyanamid Company, particularly disclose recovering the unreacted acrylonitrile by subjecting the material issuing from the reactor to a pressure reduction in a chamber kept at an absolute pressure of the order of 50 mm to 150 mm of mercury. The recovery of the acrylonitrile can be improved by entraining it by means of air or steam in the pressure release chamber. In all cases, the temperature during this operation is kept below 75.degree. C. in order to prevent the polymer from assuming an undesirable coloration; in general, the pressure reduction is carried out at a temperature of between 35.degree. C. and 60.degree. C.
From the point of view of the recovery of the unreacted acrylonitrile, this process is entirely satisfactory, more than 99% of the unreacted acrylonitrile in certain cases being recovered. This does not alter the fact that it leaves unsolved an extremely grave problem. In effect, the polymer which is obtained finally by separating the particles from the aqueous medium treated in accordance with this process still contains substantial quantities of residual monomer. The presence of this residual monomer prevents the use of the acrylonitrile polymers in numerous applications for which the polymers are entirely suitable by virtue of their intrinsic properties. Such is the case for applications where the polymer is melted so as to be molded, because the residual monomer vaporizes and pollutes the atmosphere of the shop floors. Such is also the case in the foodstuff packaging field because residual monomer is to be found in the packaging materials and presents the hazard of migrating into the packaged products and rendering them unsuitable for consumption.